Wearable Wireless Triboelectric Nanogenerator for Real‑Time Respiratory Monitoring

Abstract

Respiratory rate is a critical biomarker for detecting respiratory disorders such as obstructive sleep apnea and cystic fibrosis. Conventional clinical monitors are bulky and unsuitable for everyday use, whereas mobile health devices require lightweight, power‑free solutions. We present a waist‑wearable, wireless triboelectric nanogenerator (TENG) that senses abdominal circumference changes to produce a self‑powered electrical signal. Theoretical modeling, mechanical testing, and volunteer trials confirm that the device accurately tracks breathing rates across different rhythms, activity states, and body sizes, while delivering data wirelessly to a smartphone. This demonstrates a practical, low‑cost alternative for continuous respiratory monitoring in daily life.

Introduction

Global air quality degradation and an aging population increase the prevalence of respiratory illnesses. Continuous, non‑invasive monitoring of breathing can provide early warning for conditions such as obstructive sleep apnea syndrome (OSAS) and cystic fibrosis. Traditional monitoring equipment is often cumbersome, requires external power, and is unsuitable for long‑term home use. Recent advances in mobile networks and low‑power electronics have sparked interest in flexible, wearable health sensors. Triboelectric nanogenerators (TENGs) are ideal for this application: they are lightweight, produce high‑density energy, and are safe for skin contact. Numerous TENG‑based devices have been developed for heart rate, pulse, and general physiological monitoring, but few target respiration.

Our work introduces a smart belt that harnesses the natural expansion and contraction of the abdomen during breathing. The belt contains a sliding‑mode TENG that converts mechanical deformation into an alternating voltage without external power. The signal is amplified, digitized, and transmitted via a wireless module to a smartphone, providing real‑time respiratory data.

Methods

Device Architecture

The wearable system comprises a bilayer belt, a sliding‑mode TENG, and a wireless transmission module. The belt’s inextensible and deformable layers allow controlled sliding of the TENG’s tribo‑pair while maintaining comfort. The TENG uses a 100 µm PTFE film (negative tribo‑material) and a 30 µm nylon film (positive tribo‑material) with 50 µm copper electrodes. The 5 × 5 cm device is enclosed in a plastic sleeve to ensure consistent contact during breathing.

Figure 1. (a) Wearable belt design. (b) Mechanical test setup. (c) Wireless transmission circuitry.

Electrical Measurement

Voltage output was recorded with a Keysight B2983A electrometer. The TENG’s high impedance output required a voltage follower to match the wireless module’s input and a level‑shifting circuit to convert the bipolar signal to unipolar for ADC conversion.

Sensing Principle

Breathing induces a trapezoidal displacement of the abdominal circumference. This displacement drives the tribo‑pair to slide outward during exhalation and inward during inhalation, producing a sinusoidal voltage waveform. The four stages—intimate contact, outward sliding, pause, and inward sliding—correspond to the breathing cycle.

Figure 2. Working mechanism and four motion stages of the sliding‑mode TENG.

Theoretical Model

We model the displacement x(t) as a trapezoidal waveform and derive the output voltage V(t) using established TENG theory. The model predicts a linear relationship between peak voltage and sliding amplitude, validated by experiments (Eq. 3: V_peak = 0.01383 X_max + 0.0092).

Results and Discussion

Model Validation

The analytical prediction closely matched measured voltage during simulated breathing, confirming the model’s accuracy and guiding design optimization.

Mechanical Test

Using a stretching machine, we varied sliding amplitude from 2.5 to 30 mm at 0.5 Hz. The output voltage increased linearly with displacement, with a peak voltage of 1.2 V at 30 mm. Even a 2.5 mm displacement produced a measurable signal, indicating suitability for human breathing.

Figure 4. (a) Sensor on the stretcher. (b) Voltage vs. displacement and force. (c) Voltage at various amplitudes. (d) Peak voltage and force vs. displacement.

Real‑Time Monitoring

Volunteer tests across lying, sitting, standing, and walking demonstrated stable voltage pulses corresponding to breathing. Respiratory rates ranged from 0.54 Hz (lying) to 0.72 Hz (standing). FFT analysis confirmed accurate frequency extraction for normal, rapid, and deep breaths. A 10‑second apnea pause was also detected, showcasing potential for OSAS screening.

Figure 6. Voltage signals during different activities: (a) lying, (b) sitting, (c) standing, (d) walking.

Wireless Transmission

The wireless module, powered by a small battery, digitizes the TENG output and transmits it via Bluetooth to a smartphone. Although signal amplitude is attenuated due to the high input impedance and sampling rate, the waveform faithfully represents breathing cycles and allows accurate respiratory rate calculation.

Figure 7. (a) Wireless system setup. (b) Smartphone display of breathing waveform. (c) Processed respiratory signal.

Conclusions

We have fabricated a lightweight, battery‑free waist belt that converts abdominal breathing into a measurable electrical signal using a sliding‑mode TENG. The device accurately tracks respiratory rate across diverse activities, detects apnea events, and delivers data wirelessly to a mobile device. Its low cost, simplicity, and high sensitivity make it a promising platform for continuous home respiratory monitoring and early disease detection.

Availability of Data and Materials

All data and materials are included in the manuscript.